In the last few years visible light-emitting diodes (LEDs) based on vertical InGaN/GaN nanowires containing a p-n junction and connected collectively in parallel have for example been produced.
Generally, the term “nanowire” designates a wire the base of which is possibly as small as a few hundred nanometers in size.
By virtue of their potential intrinsic properties (good crystal quality, strain relaxation at the vertical free surfaces, good light extraction efficiency via waveguiding, etc.) nanowires are considered to be very promising candidates for alleviating the difficulties currently encountered with conventional GaN LEDs fabricated with a planar (2D) structure.
Two approaches for producing nanowire LEDs, which approaches are based on different growth techniques, have been developed at the CEA, Grenoble.
The first technological approach consists in growing GaN nanowires containing InGaN quantum wells epitaxially in an axial configuration by molecular beam epitaxy (MBE). Devices fabricated from these nanowires have yielded very exciting results in the green spectral domain. Processed chips of 1 mm2 are able to emit about 10 μW at 550 nm for a DC operating current of 100 mA.
FIG. 1 illustrates such a configuration showing nanowires NTi on the surface of a substrate 11 (typically made of silicon) making contact with a lower contact 10, the upper p-type contact being ensured by a transparent layer 12; contact redistribution is achieved via a thick redistribution contact 13. The axially structured nanowires NTi contain an n-doped zone possibly, and typically, made of n-doped GaN, an active zone ZA made of InGaN or possessing a quantum well or multi-quantum well structure and a p-doped zone possibly made of p-doped GaN.
With the molecular beam epitaxy (MBE) technique, certain nonuniformities appear because of random nucleation mechanisms, but typically an optical power of 50 nW has been obtained for a single wire emitting at 550 nm, i.e. 5 W/mm2 with one hundred nanowires emitters/mm2.
More recently, the metal organic chemical vapor deposition (MOCVD) growth technique has allowed InGaN/GaN nanowires containing a radial LED structure (core/shell configuration) to be produced.
FIG. 2 illustrates this type of configuration, in which nanowires NTn are produced on the surface of a substrate 20 covered with a nucleation layer 21 that enables lattice matching between, for example, a silicon substrate and GaN nanowires.
The structure of the nanowires comprises a photoconductive portion, made up of: a core 22 made of n-doped GaN, typically doped with a dopant density of 1019 cm-3; a quantum well structure made up of alternating layers 24, 23 that may possibly be InGaN and undoped GaN, respectively; and lastly a p-doped GaN layer 25 typically doped with a dopant density of 1019 cm-3. An insulating dielectric layer 26 is provided in order to insulate the core 22 and the upper contact. It may typically be a question of an SiO2 or SiN deposit. The upper contacts are made via a conductive upper layer 27 that is transparent to the emission wavelength of the photoconductive structure.
In this technological approach, since the LED structure has a core/shell configuration, the area of the active zone is larger than in the 2D nanowire LED approach.
This property has two advantages: it increases emissive area and decreases current density in the active zone. Complete MOCVD nanowire LED structures have been produced on a silicon substrate, and light emission in the blue spectral domain (450 nm) has been obtained for an integrated array of nanowires after technological processing.
Because of the technologies used to grow the nanowires, hundreds of thousands of wires may be produced on the surface of a chip on an area possibly, and typically, of 1 mm2.
Such novel structures, which capitalize on the emergence of nanotechnologies, have the advantage of increasing emission area and therefore the emitted light flux.
Nevertheless, since this type of LED is composed of a very substantial number of nanowires connected in parallel, it will be noted that even a very small number of defective nanowires may be responsible for poor fabrication process reproducibility and cause LED malfunction.
Specifically, if an elementary LED of a few millimeters squared comprises less than 0.1% defective nanowires, this corresponds to about one hundred nanowires that are unusable, notably because of short circuits or structural defects that lead the active zone not to function properly, being generated in the fabrication process.
Generally, nanowires may also be capable of absorbing radiation at a first wavelength shorter than their emission wavelength, thus allowing them to be controlled optically so as to emit at a desired emission wavelength.